PV Elite™

Modeling Skirt Openings in Towers

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Analyzing Openings in Skirts of Vertical Vessels

Vertical vessels that are skirt-supported typically have access openings in the skirt. These openings need to be analyzed, especially when they are of reasonable size, on tall towers, or under sizable wind/seismic loads. The information below explains skirt openings input in PV Elite.

Entering the Data

Users sometimes ask, "How do we enter the information?" Well, first indicate that the skirt element has the opening(s). Check the box to indicate this, as shown in the screenshot below:

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This will open the following dialog. Consider the various features of the data entry table. It looks like this:

image hole entry table
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Consider the various components of the table.

Number of Skirt Openings (0-10)

From the dropdown list box, choose the number of holes in the skirt. There can be up to 10 holes.

Width, Height, Center Spacing, Frame Thickness, Frame Width

Here are the dimensions:

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Layout Angle

The layout angle is the location (in plan view) around the skirt:

image layout angle
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Pitch and Ligament

In the skirt input, two very important dimensions are mentioned, and please make note of them. They are the pitch and the ligament. Here are those dimensions:

image ligament and pitch
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The pitch is the center-to-center distance between adjacent holes. The ligament distance is very important. If the ligament is too small, there is a possibility of collapse and buckling, which is a very dangerous situation. EN 13445, the code chosen as the basis for the analysis, is silent on this issue. This is because EN 13445 only gives consideration to one hole, not the multiple holes that may be analyzed by PV Elite.

The output processor (results) gives a recommended minimum ligament distance, but the user must evaluate whether this meets project requirements. The following image shows a sample of the results.

Click to enlarge.

Vessels with Multiple Skirt Elements

Note: The following notes only apply to PV Elite after the March 2007 build.

Vessels with multiple sirts raise some interesting questions.

What if the skirt is made from two separate pieces?
What happens if the access opening or hole spans the two skirts as shown in Figure 1?

Analyzing access openings and other holes in skirts can be accomplished by observing a few precautionary measures. Complications can arise when there is more than one skirt supporting the vessel. Figure 1 illustrates this:

Click to enlarge.

Figure 1

Before we discuss the hole that spans the bottom and top skirts, let us first consider some very important points:

The two skirts may have different thickness.
The two skirts may be comprised of different materials.
The two skirts may have different taper (conical) angles.

Therefore, if we have an access opening that spans both skirts, the hole cannot be considered as ONE hole in "one skirt." The analysis would be very difficult. Does this mean that a hole cannot be analyzed that spans two skirts? No, there is a simple way of analyzing the single hole that spans two skirts.

Click to enlarge.

Figure 2

Note, we have split the skirts, so each skirt is dealt with separately. This is the proper way to proceed:

In the bottom skirt, enter the hole geometry. The height of the center of the hole is located near the top of the skirt. Don't worry that the top half of the hole is outside the top node of the skirt. PV Elite can handle this without any problems. You will just receive a message that the hole breaks out of the top of the skirt. Otherwise, the analysis is correct.
In the top skirt, again, enter the hole geometry. In this case, the height of the hole is zero (or near zero). The analysis will be performed, but this time you get a message that the skirt breaks out of the bottom of the skirt. PV Elite can handle this situation.

Let's look at some other issues that may cause confusion.

The geometry of PV Elite allows entry of obround holes. For example, with an obround hole, you can use the sentry box type geometry as shown in Figure 3:

Click to enlarge.

Figure 3

The bottom of the obround opening breaks the bottom of the skirt, but PV Elite merely issues a note to that effect, but performs the analysis without any problems.